The present invention generally relates to an automobile exhaust purifying system and, more particularly, to the control of the spark timing for eventually minimizing the emission of obnoxious components of the exhaust gases emitted by the automobile engine.
Since the exhaust gases emitted by the internal combustion engines contain obnoxious components such as, for example, carbon monoxide (CO), hydrocarbon (HC) and nitric oxides (NOx), improvement had been, and is currently being, made on the fuel suction system, the fuel supply system, the ignition system, the exhaust system and/or some other control systems of the engine for the purpose of minimizing the emission of the obnoxious exhaust gas components. Of these numerous efforts hitherto rendered for minimizing the emission of the obnoxious exhaust gas components, the use of a catalytic converter on the exhaust system of the engine is largely practised.
As is well known to those skilled in the art, the catalytic converter generally comprises a heat-resistant vessel containing a mass of particles or pellets of one or a mixture of catalytic materials including, for example, platinum, vanadium and manganese and is so designed as to remove, by combustion or adsorption, the obnoxious exhaust gas components during the passage of the exhaust gases therethrough. In practice, the catalyst in the catalytic converter must be heated to a predetermined temperature or higher in order for it to exhibit the maximized exhaust gas purifying efficiency.
In the limited environments of the automobile utilizing the catalytic converter, the catalyst in the catalytic converter must be heated to a required temperature or higher in a minimized period of time particularly after the cold start of the engine so that the time during which the catalyst fails to work efficiently because of the low temperature can be minimized. This preheating of the catalyst is generally known as a warm-up of the catalyst and depends on, inter alia, heat carried by the exhaust gases emerging from the engine. The higher the temperature of the exhaust gases, the readier the warm-up of the catalyst. For increasing the temperature of the exhaust gases with a view to facilitating the catalyst warm-up, some methods are currently employed, including the control of the spark-plug ignition timing such as disclosed in, for example, the Japanese Laid-open Utility Model Publication No. 57-18771, first published on Jan. 30, 1982.
According to the above mentioned publication, the control of the spark advance is interrupted temporarily during a particular period while the engine is being warmed up, to warm up the catalyst, whereby the spark advance is controlled in dependence on the temperature of the exhaust gases or the temperature inside the catalytic converter.
In general, it is well known that, when the spark-plug ignition timing, that is, the spark timing, is retarded, the temperature of the exhaust gases subsequently emerging from the engine can be increased. In the case of the spark timing control system disclosed in the above mentioned Japanese publication, however, the spark timing is retarded by suspending the control of the spark advance, thereby to produce the elevated temperature of the exhaust gases necessary to warm up the catalyst. This prior art system appears to be satisfactory so far as the warm-up of the catalyst is concerned and, however, it has failed to take the time-dependent characteristic of the catalyst into consideration as will now be discussed.
As is well known to those skilled in the art, the characteristics of the catalyst used in the catalytic converter for use in the automobile exhaust system deteriorate with increase of the operating time, i.e., the cumulative time during which the catalytic converter is actually utilized. This in turn results in the corresponding reduction in capability of the catalyst to substantially purify the automobile exhaust gases. The reduction in capability of the catalyst adversely affects the manner in which the temperature of the catalyst in the catalytic converter is increased. More specifically, referring to the graph shown in FIG. 1 of the accompanying drawings wherein the axis of abscissas represents the passage of time from the cold start of the automobile engine and the axis of ordinates represents the temperature of the catalyst used in the catalytic converter, the catalyst when fresh is heated incident to the operation of the engine in such a manner as shown by the solid line a whereas the same catalyst when having been used for a long time with the purifying capability consequently reduced exhibits the increase in temperature in such a manner as shown by the chain line b. As can readily understood from the graph of FIG. 1, when the catalyst becomes aged in the sense that the exhaust gas purifying capability has been reduced, not only the rate of increase of the temperature of the catalyst, but also the maximum temperature achieved by the aged catalyst for a given temperature of the exhaust gases are reduced as compared with those afforded by the same catalyst when it was fresh.
In view of the above, where the warm-up of the catalyst relies on the increased temperature of the exhaust gases resulting from the suspended control of the spark advance effected during the warm-up of the engine such as achieved in the previously mentioned prior art, there is a problem in that, although the catalyst works satisfactorily in substantially purifying the exhaust gases when it is relatively fresh, it will, when aged, exhibit the reduced purifying effect and require a relatively long time to pass before it attains the required temperature.